1. Technical Field
The present invention relates to isolation circuitry. More particularly, the present invention relates to an improved solid state isolation device such as used in telephone line interface equipment.
2. Description of the Prior Art
Devices that isolate user telephone equipment from telephone networks are required by government regulatory agencies, such as the U.S. Federal Communications Commission, to protect the telephone networks from anomalies related to the user equipment, for example as might result from the use of nonstandard or defective equipment or the inadvertent coupling of line voltages into the telephone network. Isolation also protects the user and user equipment from telephone network-related anomalies, such as voltage surges due to power supply fluctuation and lightning. A discussion of the state of the art with regard to isolation devices for telephone networks is provided in D. Wilkison, D. Lee, Solid State Isolation Device Using Opto-Isolators, U.S. Pat. No. 5,245,654 (14 Sep. 1993).
FIG. 1 is a block schematic diagram of an isolation circuit disclosed in U.S. Pat. No. 5,245,654. The figure shows a state of the art isolation circuit, also referred to as a direct access arrangement ("DAA") 10, that interfaces a user device 12, such as a modem, to a telephone network. The network is shown as a pair of signal lines 15T and 15R, often referred to as "tip" and "ring", respectively. The DAA 10 couples a pair of analog channels--transmit and receive--to the telephone network, while providing a high level of DC isolation between the user device and the lines 15T and 15R. The DAA also provides surge protection, rectification, off-hook detection, and ring detection.
The DAA 10 includes an isolation circuit 20 that is coupled to a user device 12 via transmit and receive lines 22, 23, and a line coupler 25 that is connected to the lines 15T and 15R. The isolation circuit and the line coupler communicate via a pair of signal lines 35, 37, that are also designated as L+ and L-, and which generally correspond to tip and ring.
The isolation circuit includes a transmit optical isolator circuit 50, a receive optical isolator circuit 52, and a hybrid 55. The optical isolator circuit 50 is disposed between the user device transmit channel and the hybrid; and the optical isolator circuit 52 is disposed between the receive channel and the hybrid. The optical isolator circuits 50, 52 serve to communicate analog signals across an isolation barrier, shown in the figure as a dashed line 57, while preventing an electrical connection across the barrier. The hybrid 55 interfaces the two-conductor line circuit, L+ and L-, to separate transmit and receive channels, which typically consist of four conductors, to permit full duplex operation.
The many advantages of using optical isolation instead of isolation transformers are discussed in U.S. Pat. No. 5,245,654, which is incorporated herein by this reference thereto. It has been found in practice that the use of optical isolation techniques to interface user equipment to a telephone network has presented unique problems and limitations, especially when such approach is applied to telephone networks having disparate operating parameters and ranges, such as are encountered in the world's many different telephone standards and systems.
Such problems and limitations include, for example:
1. Signal distortion and insensitivity to low signal levels resulting from low loop current, lack of complex impedance compensation/termination for diverse telephone system parameters, and/or internal circuit losses; PA1 2. Limited physical interface capability, for example in applications requiring 3-wire ring connectivity; PA1 3. Poor common mode rejection ratios in the transmit and receive paths; and PA1 4. Lack of compensation for manufacturing variances in circuit components, especially variances in the opto-isolators.
It would be advantageous to improve known telephone network optical isolation circuits, such that the various problems and limitations attendant therewith are overcome.